1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus.
2. Description of the Related Art
Recently, high intensity discharge lamps (HID lamps) have been developed as a lighting source of video devices such as projectors, auto headlights, and display lightings. Such discharge lamps are characterized in that high intensity can be obtained with a low power as compared with conventional lighting sources and in particular, are highly promising as a lighting source of projectors and auto headlights.
For lighting such discharge lamps, there have been proposed various types of discharge lamp lighting apparatuses. The proposed discharge lamp lighting apparatuses can be classified into three depending on their starting methods, i.e., a direct current starting method (Japanese Unexamined Patent Application Publication No. 2001-273984), a low frequency starting method, and a high frequency starting method (Japanese Unexamined Patent Application Publication No. 2008-59806). The direct current starting method is a method in which a high-voltage pulse is generated for starting with a constant direct voltage applied between electrodes of a discharge lamp and the direct voltage is maintained for a given length of time after starting the discharge. The low frequency starting method is a method in which a high-voltage pulse is generated with an alternating voltage of a frequency as low as a few hundred Hz applied between electrodes of a discharge lamp and the frequency is maintained after starting the discharge. The high frequency starting method is a method in which a high-voltage pulse is generated with an alternating voltage of a frequency as high as a few dozen kHz applied between electrodes of a discharge lamp and the frequency is maintained after starting the discharge.
The above starting methods have different driving methods but are similar in having a DC/DC converter at the front stage of a circuit component for driving a discharge lamp (discharge lamp driver). The DC/DC converter is adapted to convert an input direct voltage, which is obtained such as by rectifying and smoothing a commercial AC source, to a direct voltage of a different voltage value than the input direct voltage by switching and then output the converted direct voltage, while the discharge lamp driver is adapted to convert the direct voltage, which is supplied from the DC/DC converter, to a voltage suitable for driving a discharge lamp.
When using the DC/DC converter, since the interelectrode voltage (tube voltage) of a discharge lamp greatly varies between the starting operation and the stationary operation, the pulse width is controlled, for example, in order to stabilize power consumption of the discharge lamp regardless of such a variation. For controlling the pulse width, there is adopted a pulse width control IC containing a reference pulse oscillator. The pulse width control IC receives a pulse width control signal from a microprocessor (MPU) and generates a pulse width controlled switching control signal using waveform of an oscillating signal of the contained reference pulse oscillator. The switching control signal is supplied to the DC/DC converter and pulse width controlled switching operation is performed by the DC/DC converter. The DC/DC converter performs the switching operation at a high switching frequency equal to or greater than 50 kHz.
The DC/DC converter has a smoothing circuit such as of a capacitor input type on its output side, thereby converting a switching output to a direct voltage. Since the DC/DC converter performs the switching operation at a high switching frequency equal to or greater than 50 kHz, as described above, the direct voltage contains a ripple corresponding to the switching frequency and the ripple is supplied to the discharge lamp driver.
The discharge lamp driver has a function of converting a direct current, which is supplied from the DC/DC converter, to an alternating voltage suitable for driving the discharge lamp during stationary operation (DC/AC inverter). The switching frequency of the DC/AC inverter is generally set at a value lower than the frequency of the DC/DC converter. In the case of the low frequency starting method, for example, it is about 100 to 150 Hz. Accordingly, the ripple contained in the direct voltage of the DC/DC converter appears in the output waveform of the DC/AC inverter as it is, whereby a ripple current flows through the discharge lamp.
The discharge lamp lighting apparatus, which has the above basic configuration and operates as described above, has the following incidental problems regardless of the starting methods.
(1) Problems Related to Ripple Current
The ripple current has to be reduced as much as possible because it affects the quality of light from the discharge lamp and the lifetime of the discharge lamp. In conventional discharge lamp lighting apparatuses, the reduction of the ripple current has been achieved either by increasing the capacity of a capacitor or the inductance of an inductor in the smoothing circuit attached to the DC/DC converter or by increasing the switching frequency of the DC/DC converter.
However, increasing the capacity of a capacitor or the inductance of an inductor in the smoothing circuit results in increasing the size and cost of the discharge lamp lighting apparatus. On the other hand, increasing the switching frequency of the DC/DC converter results in increasing the heat generation and temperature of the DC/DC converter.
(2) Problems Related to Microprocessor
In discharge lamp lighting apparatuses, the microprocessor for controlling the apparatus as a whole is sometimes influenced by some external factor, so that the program may run out of control into a failure mode where the electric potential is fixed at all the output terminals of the microprocessor. If the behavior at this time is analyzed by FEMA (Failure Mode and Effect Analysis), the apparatus itself may go out of control. Accordingly, heretofore, a monitoring circuit has been provided outside the microprocessor to monitor the runaway and stop the operation of the discharge lamp lighting apparatus upon detection of the runaway. This increases the number of elements constituting the discharge lamp lighting apparatus, hindering miniaturization and cost reduction.
(3) Problems Related to Oscillation Frequency of Pulse Width Control IC
In conventional discharge lamp lighting apparatuses, a pulse width control IC containing an oscillator is provided with external resistors and capacitors for setting the oscillation frequency of the oscillator contained in the pulse width control IC. However, commercially available resistors and capacitors generally have a margin of error of a few percent. The accuracy of the oscillation frequency thus set depends on the product of error margin of the external resistors and capacitors and the reference voltage set inside the pulse width control IC. Accordingly, there has been a limit to increasing the accuracy of the oscillation frequency.